1. Technical Field
This invention relates generally to a system for controlling a switch-reluctance (SR) motor, and more particularly, to a system for particularly controlling the operation of an SR motor between a multi-phase mode and a reduced phase operating mode.
2. Discussion of the Related Art
Switched reluctance (SR) machines have been the subject of increased investigation due to their many advantages, which makes them suitable for use in a wide variety of situations. An SR machine operates on the basis of varying reluctance in its several magnetic circuits. In particular, such machines are generally doubly salient motors--that is, they have teeth or poles on both the stator and the rotor. The stator teeth have windings which form machine phases of the motor. In a common configuration, stator windings on diametrically opposite poles are connected in series to form one machine phase.
When a stator phase is energized, the closest rotor pole pair is attracted towards the stator pole pair having the energized stator winding, thus minimizing the reluctance of the magnetic path. By energizing consecutive stator windings (i.e., machine phases) in succession, in a cyclical fashion, it is possible to develop torque, and thus rotation of the rotor in either a clockwise, or counter-clockwise direction.
As further background, the inductance of a stator winding associated with a stator pole pair varies as a function of rotor position. Specifically, the inductance varies from a lower level, when a rotor pole is unaligned with a corresponding stator pole, to an upper or maximum level when the rotor pole and stator pole are in alignment. Thus, when the rotor pole rotates and sweeps past a stator pole, the inductance of the stator winding varies through lower-upper-lower inductance levels. This inductance-versus-rotor position characteristic is particularly relevant for controlled operation of the motor. Specifically, current flowing through the stator winding must be switched on prior to (i.e., advanced), and maintained during the rising inductance period to develop positive torque. Since positive phase current during the decreasing inductance interval produces a negative or breaking torque, the phase current must be switched off before this interval occurs to avoid generating negative torque. Accordingly, rotor position sensing is an integral part of a closed-loop variable-reluctance motor drive system so as to appropriately control torque generation.
Further, such motors may be operated in a multi-phase mode of operation, which is desirable when a relatively large load is driven by the motor. However, in some instances, the motor may be operated for a period of time in a low load condition (e.g., no load, or lightly loaded--hereinafter a "Low Load Condition") . When this occurs, the speed of the motor may rise rapidly. Conventional control methods and devices have continued to operate the motor in a multi-phase mode in this low load condition (i.e., all of the machine phases being sequentially energized to effect rotor rotation). This mode of operation, however, is less than optimally efficient. Particularly, since only a low load is being driven, energizing current in each of the multiple phases goes to a low level, which, for SR motors, may generate less torque per unit current than when energized at a higher current level.
Accordingly, there is a need to provide an improved system for controlling a switched reluctance machine that minimizes or eliminates one or more of the problems as set forth above.